Bearing Capacity of Working Platforms using Distinct Layout Optimization Method

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(1)476. Geotechnical Safety and Risk V T. Schweckendiek et al. (Eds.) © 2015 The authors and IOS Press. This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License. doi:10.3233/978-1-61499-580-7-476. Bearing Capacity of Working Platforms Using Distinct Layout Opitimization Method Lech

(2) . Department of Geotechnics, Geology and Maritime Engineering, Gdansk University of Technology, Poland. Abstract. Bearing capacity of the ! " # ! $ % ! & ' (fferent failure modes are consider! $ # $ )(%* !+

(3) +#+ +! $! ,$ $ ! ' (!! !#+ $! !! ,!-! $# +! +#!

(4) ! ! # +! ! $! +! !+ +! $,!' .+! ,$ $ed shear strength with depth was also taken in #!' .+! # $! ! !! !!!' .+! ##$! ,! capacity was compared to analytical solutions and !#! /' !# ! +! ! on soft clay with undrained shear strength lower + 01 2

(5) + #! #-!! , +!! !#!' .+! solution obtained with DLO method was compared to bearing capacity calculation using Plaxis and 3

(6) # !$# !+' Additionally some analysis was performed for a real platform of # #' !!

(7) +! !+ deformation parameters of the platform material and the subgrade were determined with a !! ! )(4.*' .+ approach permits to determine both strength and deformation parameters of the platform material and soft subgrade in one test' Keywords. Limit sta!

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(9) $! +! !+. 1. Introduction .+! design of working platforms can be considered as a bearing capacity problem of !! ' .+! upper layer – platform made up from compacted cohesionless material – is constructed on weak subgrade to permit traffic of +!- #+!' .+! ! $,! ,! !! #+!-! #+!!' .+ ! considers the first layer of medium sand and the soft layer from clay' .+! +#! +!

(10) +! ##! and strength of the platform material have to assure safe traffic of heavy equipment like drilling

(11) heavy tracks or compactors and the proper execution of piling or soi -!! ' Different methods exist to estimate bearing capacity of the shallow foundation on such !! ' .+! $ ,! # !+ !!! / 470 or & (2012) recommendations for contractors !!#+# ' .+! !# one uses limit state approach' !!

(12) % ! & ! was applied to find the kinematically admissible estimation of the $, ,! ##' .+!. third group of methods addressed in this paper considers the use of <! !! 4! )<4* '. 2. Geometry and Soil Conditions 2.1. Geometry of the Problem .+! !! a wider research program concerning the design of working ' !!

(13) +! ! #$sed on the bearing capacity of the platform resting on very soft normally consolidated and overconsolidated $,!' !# $ + parameters assumed using in-$ !' A typical !! # ! $!' .+! !! #+!! +! ,! -! <' >' .+! +#! the upper cohesionless layer is -,!

(14) +! +! !! +! ! subgrade is kept unchange' $! + the track with width of ?1'@

(15) transmits a uniform load of 77 kPa from tracked plant on caterpillar +! $,'.

(16) L. Bałachowski and K. Białek / Bearing Capacity of Working Platforms Using Distinct Layout Opitimization Method. 2.2. Soil Parameters .+! !+ ! !! +! real working platform were estimated using dilatometer test results' $#+ #+ ! both strength (3'

(17) #u) and deformation soil parameters to be determined in one penetration !' .+! !!#-! ! internal friction for the platform material and the undrained shear strength determined for the soft subgrade ##$! $ +! $! 4#+! ! ' (2001) ! -! <' 0' .+! ! ! friction is estimated in the range from 35° to 42° and the undrained shear strength is from 5 to 16 kPa

(18) +#+ corresponds to very soft to #' .+! -!! #! $$ equal to 40 MPa for the platform material and 2 MPa for the $,!' .he soil parameters admitted in the analysis are based on the (4. test results'. 477. with depth were assumed with cu equal to 10 and 01 2' Additionall

(19) two other models of undrained shear strength distributions with depth !! $!' .+! ! #! +! shear strength increasing with depth at 2 kPa/m and the second one is described with cu decreasing with !+ 0 2N' .+! $! +! !+ increasing with depth is typical for normally #!

(20) +! ,$ #u #! -!#! '. Figure 3. Undrained shear strength distribution in weak subgrade'. 3. Description of the Calculation Methods Used. Figure 1. General scheme of the working platform' angle of internal friction ' [°] 32. 34. 36. 38. 40. 42. 0 angle of internal friction - ' 0,4. z [m]. 0,8 1,2 weak subgrade - cu. 1,6 2 4. 8. 12. 16. 20. 24. undrained shear strength cu [kPa]. Figure 2. (4. ! !$ in the platform and the subgrade'. . into account the estimated undrained +! !+ +! $, )<' 0* our different distributions of cu were analyzed in this study (<' J)' . # ,$s of cu. .+! ! ##$ !#,! / Q@1 guide is based on the analysis developed by Meyerhof (1974) for a footing punching through a strong platform material overlying a weak $,!' .+! ,! $#+ $! represents a major simplification of the actual field situation and is semi-!# #+#!' Acc / Q@1 it is possible to calculate +! !#! +#! -!

(21) foundation shape and shear strength parameters of the platform (3’) and the soft subgrade (cu*' is also possible to check the platform thickness when geosynthetic reinforcement is introduced $! +! !' .+! $! recommendations is restrained to the soft subsoil with cu !S$ ! 01 2' .+$ / 470 recommendations cannot be applied to very soft $,'. inematically admissible solution of bearing capacity can be determined $ % ! & ' !!s the ultimate limit ! )T% * $ +! #$ analysis technique - Discontinuity Layout )(%*' .+! procedure is implemented to identify the critical $#! +! +! $! ##$' .+! ! of rigid sliding blocks in plane strain condition together with inter-,# #! ! ,!' .+! ##$! ,!aring capacity is a.

(22) 478. L. Bałachowski and K. Białek / Bearing Capacity of Working Platforms Using Distinct Layout Opitimization Method. kinematically admissible solution and forms the $! ,$ +! !' . !! #+! ! ,!' +! V<# oad” approach is used the load at failure is obtained

(23) so it is possible to calculate the margin of safety ##! +! ! ' V<# strength” approach is chosen the strength !!

(24) '!' )3’) and c’ or cu

(25) ! -! , a #! # $ $!' + ! # get a margin of safety concerning the effective or !+ !! +! $,' .+! software allows applying the partial coefficients concerning both actions and strength parameters in order to meet the requirements of calculations ## -@' he present analysis all partial coefficients concerning actions and !+ !! !! $! !S$ >' .+ +! # !+ $! + ! determine the bearing capacity of the working ' Bearing capacity of shallow foundation can ,! ##$! $ <4 ogram like 2%X

(26) Y! )01>J*' .hen 3

(27) c reduction procedure is applied and the margin of safety concerning the soil strength parameters can be established

(28) Z!!! )0110*' +!! !+ the analyses were made assuming strip foundation and plane strain #' .+! Mohr-Coulomb model was used !#,! $! #!' .+! material was considered as drained and weak $,! $! $!' .+! ## area between caterpillar and working platform was characterised by the boundary element having zero thickness and geotechnical !! !$#! , [1\ '. observed in <' Q changes progressively with platform thickness' For (h/B) not larger than 1 the punching failure in platform material is followed by the generalized failure in the soft $,' +! !-! +#! +! increases the failure pattern doesn’t penetrate the soft layer further but is constrained within the interface betwe! !' .+! !!! $! !#+ +! relative thickness of the platform (h/B) equalling 2 is given in <' [' + #! +! $! ! #! +! ! '. 4. Calculations 4.1. Results obtained with DLO .+! ##$ !! !! $ V<# ] V<# !+] #+!' the first step the values of internal friction angle for platform material in the range from 32° to 38° and cu=20 kPa !! $!' .+! !# !#+ failure and ##$! # ! -! <' Q 5 for 3'=35° and different relative thickness of the platform' .+! punching failure mechanism. Figure 4. Punching failure for (h/B) from 1'[ >'['. .+! ##$! # load is given in <' { a function of relative thickness of the platform )+N*' < )+N* -$! $ 1'@[ the same factor on load is obtained regardless of the effective friction angle of the platform.

(29) L. Bałachowski and K. Białek / Bearing Capacity of Working Platforms Using Distinct Layout Opitimization Method. 6 5. '=32° '=35°. 4. Factor on load. ma!' ! + +! $#+ $! occurs in the platform with generalized failure in the soft subgrade and the bearing capacity of the working platform is mostly governed by the $! +! !+ +! ! $,!' For relative thickness of the platform (h/B) larger + 1'@[ +! ##$! # #!! with the effective angle of internal friction of the !' .+! # reaches its |$ -$!

(30) ,! a function of the internal friction angle 3'

(31) for the relative thickness of the platform (h/B* ! + 0' +$ ,! ! out that for higher values of internal friction angle some side effects were observed for higher platform thickness with sliding lines approaching the lateral boundaries of the

(32) +#+ !d to overpredicted values of the calculated factor on load' $#+ #!s the calculations were !!! $ !! ' # fety coefficient for shallow foundation equalling 2 is applied

(33) one can determine the necessary relative thickness of the platform to satisfy bearing capacity #' .+ !-! +#! )+N* +$ !|#!! >'> for 3}?J0~

(34) 1' 3}?J[~ 1'[ 3}?J~'. 479. '=38°. 3 2. cu=20 kPa. 1 0 0. 0,5. 1. 1,5. 2. 2,5. h/B. Figure 6. Factor on load -' !-! +#! of the platform !! -$! ! # !'. +! !| + $ the different schemes for undrained shear strength distribution were taken into consideration )!! <' J)' . constant profiles of cu were analyzed with undrained shear strength equalling 10 kPa and 20 2'

(35) the profile with cu starting at 10 kPa and increasing at 2kPa/m and the cu profile starting at 20 kPa and decreasing at 2kPa/m with depth were taken into account' . different angles of internal friction of the ! !! #!!

(36) '!' 3’ equalling 35° an J~' .+! ##$ results of factor on load for 3’=35° are given in <' @' One can notice a considerable difference between the factors on load obtained for cu equalling 10 kPa 01 2' Only a small effect of the undrained shear strength increase or decrease with depth on the factor on load ,!-!' .+! !#! relative thickness of the platform to satisfy bearing capacity condition shoul !|#!! >'[ cu? >1 2 1' #u?01 2' 4,5. Figure 5. !# !#+ of failure for (h/B)=2'. 4 cu=20kPa. 3,5. cu -2kPa/m. Factor on load. 3. cu=10kPa 2,5. cu +2kPa/m. 2 1,5 1. '=35°. 0,5 0 0. 0,5. 1. 1,5. 2. 2,5. h/B. Figure 7. Factor on load vs' relative thickness of the platform for different cu #+!!'.

(37) 480. L. Bałachowski and K. Białek / Bearing Capacity of Working Platforms Using Distinct Layout Opitimization Method. .+e results obtained for factor on strength calculated with different angles of internal friction and the undrained shear strength of the weak subgrade cu=20 kPa constant in the soil profile are given in <' ' .+ # #!! with the relative thickness of the platform and with the effective angle of internal friction of the platform material

(38) +!-! some fluctuations of the results can be observed' .+! ! obtained with factor on strength cannot be however interpreted in such straightforward manner as using the factor on load approach' the next chapter the results will be compared to analysis obtained with Plaxis'. shape of the foundation were assumed equal to 1 in order to compare the results with previous methods' .+! !! !! the effective angle of internal friction 3’=38° and the undrained shear strength of the weak subgrade cu?01 2' According to restrictions imposed by / $! he calculation was made for the relative thickness of the platform (h/B) not larger + >'[' .+! $! !#+ ++! values of relative thickness (h/B) differs from the punching failure scheme involved / !#!

(39) +! ##$ methods should be used in such #!'. 4.2. Results according to Plaxis. 5. Discussion of results. .+! ! !! !! +! ! friction angle 3’=38° and the undrained shear strength of the weak subgrade cu=20 kPa for - !-! +#! +! ' .+! 3

(40) c reduction procedure was applied at the end of calculation of bearing capacity of strip foundation and the factor on strength was !!!'. .+! #s on strength determined with % ! &O and Plaxis are #! )<' ) for the effective angle of internal friction 3’=38° and the undrained shear strength of the weak subgrade cu?01 2' Z! !$ for both methods were obtained with slightly higher & factor on strength for the relative thickness of the platform (h/B) larger than 1' 1,9. 1,8. '=32°. 1,7. '=35°. 1,6. '=38°. 1,8 1,7. Factor on strength. Factor on strength. 1,9. 1,5 1,4 1,3. 1,6 1,5 1,4. GEO. 1,3. plaxis. 1,2. 1,2 1,1. 1,1. cu=20 kPa. 1. 1 0. 0,5. 1. 1,5. 2. 2,5. h/B. Figure 8. Factor on strength vs' relative thickness of the platform for different angles ! #'. 4.3. BRE method According to / recommendation it is possible to estimate the necessary thickness of the -! #' < the purpose of this study the inverse approach was used and the failure load was determined for the assumed +#!' All the coefficients concerning the applied load and the. 0. 0,5. 1. 1,5. 2. 2,5. h/B. Figure 9. <# !+ ## & 2| ##$'. <

(41) +! ,! ## $ on a working platform was compared for % ! & / !+ )<' >1*' .+! !$ are given for undrained shear strength of the soft subgrade equalling 01 2' Due to limitation of the / !+ he results !-! +#! )+N* ++! + >'[ are plotted' < )+N* ! + 1'@[ +! results are almost the same regardless of the calculation method and the angle of internal # +! !' < ++! )+N*.

(42) L. Bałachowski and K. Białek / Bearing Capacity of Working Platforms Using Distinct Layout Opitimization Method. ratios the influence of shear strength of the platform material is getting more important as +! $! !#+ #+ !-!' One can notice that the bearing capacity obtained + % ! & systematically higher + + ##$! ## /' !! +! # + % ! -! a kinematically admissible solution which forms the upper bound for the bearing capacity ,!' 450 '=35° LimitState GEO Unity. 400. '=38° LimitState GEO Unity. Bearing capacity. 350. '=35° BRE 300. '=38° BRE. 250 200. 481. higher (h/B) values the generalized failure !#+ + ! ! #!' #! + +! of safety for the bearing capacity problem is sensitive to $! +! !+

(43) +!-! +! $!#! of cu distribution with depth according to the admitted schemes was negligible' $ + +! % ! & method is quite sensitive to the mesh coarseness and to the size of the domain' .+! $!#! boundary effects on the obtained solution should be thus carefully examined in order to get reliable results' .+! attempt $! (4. +! estimation of the soil parameters for the platform and the subgrade in one penetration test seems to be promising' .+ #+ +$ ,! verified on other trial fields using other in-situ and , !'. 150 100 0,2. 0,7. 1,2. 1,7. 2,2. 2,7. h/B. Figure 10. Bearing capacity of the working platform loaded by strip foundation ## / & '. 6. Conclusions .+!! !! !+ !! $! !!! the bearing capacity of the strip foundation on two-!! ' .+! was focused on the behaviour of the platform on very soft to soft #' .+! applied calculation methods should be used with respect to different failure mechanisms ,!-!' .+!! !#+ !! n the relative thickness of the platform but also on the effective angle of internal friction of the platform and the undrained shear strength of the soft subgrade as well' Analysis of the failure mechanisms for different relative thickness of the working platform on soft subgrade confirms that the punching failure in the platform material can ,! ,!-! )+N* ! + >'[' <. References Y! ' )0013) Analysis of the punching failure mechanism

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